Atomistic and continuum scale modeling of functionalized graphyne membranes for water desalination.

Investigating the microstructures and surface features of seawater RO membranes and the dependencies of fouling resistance performances

Commercial seawater reverse osmosis (SWRO) membranes were coated with iron nanoparticles (FeNPs) and biofouled with a bacterium strain isolated from the Sea of Cortez, Mexico. This strain was selected and characterized, as it was the only cultivable strain in pretreated seawater. Molecular identification of the strain showed that it belongs to Bacillus halotolerans MCC1. This strain was Gram positive with spore production, and was susceptible to Fe+2 toxicity with a minimum inhibitory concentration of 1.8 g L−1. Its biofouling potential on both uncoated and FeNP coated reverse osmosis (RO) membranes was measured via biofilm layer thickness, total cell count, optical density and organic matter. The FeNP-coated RO membrane presented a significant reduction in biofilm cake layer thickness (>90%), total cells (>67%), optical density (>42%) and organic matter (>92%) with respect to an uncoated commercial membrane. Thus, Bacillus halotolerans MCC1 shows great potential to biofoul RO membranes as it can pass through ultrafiltration membranes due to its spore producing ability; nonetheless, FeNP-coated membranes represent a potential alternative to mitigate RO membrane biofouling.

Permeation through membrane with pores is important in the choice of materials for filtration and separation techniques. Here, we report by the molecular dynamics simulations that a single-layer graphyne membrane can be impermeable to salt ions, while it allows the permeation of water molecules. The salt rejection and water permeability of graphyne are closely related to the hydrostatic pressure, type of graphyne membrane, and the salt concentration of solution, respectively. By analyzing hydration shell structure, we found that the average coordination number of ions plays a key role in water purification. Our calculation showed that the salt rejection of the graphyne-3 membrane is the best and it can keep an ideal rate of 100% in consideration cases. In comprehensive evaluation of both salt rejection and permeability, the graphyne-4 is a perfect purification membrane. To sum up, our results indicated that the graphynes (graphyne-3 and -4) not only have higher salt rejection but also possess higher water permeability which is several orders of magnitude higher than conventional reverse osmosis membranes. The single-layer graphyne membrane may have a great potential application as a membrane for water purification.

Membrane separation processes are energetically efficient compared to the other techniques such as cryogenic distillation and gas adsorption techniques. It is well known that a membrane's permeance is inversely proportional to its thickness. Regard to its single atom thickness and its mechanical strength, nanoporous graphene has been proposed as a very promising candidate for highly efficient gas separation applications. In this work, using classical molecular dynamics, we report the separation performance of such membrane in a molecular-sieving process as a function of pore size and chemical functionalization of pore rim. To investigate the membrane separation capability, we have calculated the permeance of each gas molecule of the considered binary mixtures through the membranes and therefore the separation selectivity. We investigated the separation performance of nanoporous graphene for CO 2 /N 2 , H 2 /CH 4 and He/CH 4 with 50:50 proportions of each component and the separation selectivity has been calculated. We also calculated the potential of the mean force to characterize the energy profile for gas transmission. The separation selectivity reduced by increasing the pore size. However, presence of chemical functionally pores in the membrane increased the separation selectivity. Furthermore, the gas permeance through nanoporous graphene membranes is related not only to transport rate to the graphene surface as well as kinetic diameters but also to molecular adsorbed layer which is formed on the surface. The flux of molecules through the nanopores is also dependent on pore chemistry which is considered as gas-pore interactions in the molecular simulations and can be a sizable factor in simulation in contrast to experimental observations. This study suggests that nanoporous graphene could represent a suitable membrane for gas separation.

Investigation of aqueous and organic co-solvents roles in fabricating seawater reverse osmosis membrane

To better understand biofouling of seawater reverse osmosis (SWRO) membranes, bacterial diversity was characterized in the intake water, in subsequently pretreated water and on SWRO membranes from a full-scale desalination plant (FSDP) during a 9 month period. 16S rRNA gene fingerprinting and sequencing revealed that bacterial communities in the water samples and on the SWRO membranes were very different. For the different sampling dates, the bacterial diversity of the active and the total bacterial fractions of the water samples remained relatively stable over the sampling period whereas the bacterial community structure on the four SWRO membrane samples was significantly different. The richness and evenness of the SWRO membrane bacterial communities increased with usage time with an increase in the Shannon diversity index of 2.2 to 3.7. In the oldest SWRO membrane (330 days), no single operational taxonomic unit (OTU) dominated and the majority of the OTUs fell into the Alphaproteobacteria or the Planctomycetes. In striking contrast, a Betaproteobacteria OTU affiliated to the genus Ideonella was dominant and exclusively found in the membrane used for the shortest time (10 days). This suggests that bacteria belonging to this genus could be one of the primary colonizers of the SWRO membrane. Knowledge of the dominant bacterial species on SWRO membranes and their dynamics should help guide culture studies for physiological characterization of biofilm forming species.

A nanofibrous composite reverse osmosis (RO) membrane with a polyamide barrier layer containing interfacial water channels was fabricated on an electrospun nanofibrous substrate via an interfacial polymerization process. The RO membrane was employed for desalination of brackish water and exhibited enhanced permeation flux as well as rejection ratio. Nanocellulose was prepared by sequential oxidations of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and sodium periodate systems and surface grafting with different alkyl groups including octyl, decanyl, dodecanyl, tetradecanyl, cetyl, and octadecanyl groups. The chemical structure of the modified nanocellulose was verified subsequently by Fourier transform infrared (FTIR), thermal gravimetric analysis (TGA), and solid NMR measurements. Two monomers, trimesoyl chloride (TMC) and m-phenylenediamine (MPD), were employed to prepare a cross-linked polyamide matrix, i.e., the barrier layer of the RO membrane, which integrated with the alkyl groups-grafted nanocellulose to build up interfacial water channels via interfacial polymerization. The top and cross-sectional morphologies of the composite barrier layer were observed by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) to verify the integration structure of the nanofibrous composite containing water channels. The aggregation and distribution of water molecules in the nanofibrous composite RO membrane verified the existence of water channels, demonstrated by molecular dynamics (MD) simulations. The desalination performance of the nanofibrous composite RO membrane was conducted and compared with that of commercially available RO membranes in the processing of brackish water, where 3 times higher permeation flux and 99.1% rejection ratio against NaCl were accomplished. This indicated that the engineering of interfacial water channels in the barrier layer could substantially increase the permeation flux of the nanofibrous composite membrane while retaining the high rejection ratio as well, i.e., to break through the trade-off between permeation flux and rejection ratio. Antifouling properties, chlorine resistance, and long-term desalination performance were also demonstrated to evaluate the potential applications of the nanofibrous composite RO membrane; remarkable durability and robustness were achieved in addition to 3 times higher permeation flux and a higher rejection ratio against commercial RO membranes in brackish water desalination.

Performance evaluation and fouling characterisation of two commercial SWRO membranes

Impact of pilot-scale PSF substrate surface and pore structural properties on tailoring seawater reverse osmosis membrane performance

Reverse osmosis (RO) technology is currently the most progressive, energy-saving and efficient membrane separation technology . Meanwhile, graphene becomes a promising candidate for fabricating the RO membranes in water desalination due to its high salt rejection and water flux. The concept of “temporal selectivity” is first proposed in our previous work in terms of the time difference between the penetration time of an ion passing through the pore and the tangential slipping time for the ion sliding across the pore. Nevertheless, the temporal selectivity mechanism of multilayered graphene membrane remains ambiguous. In this paper, the RO process of saltwater through porous graphene column RO membrane is studied by using molecular dynamics (MD) simulations method, and the effects of rotating angular velocity and the thickness of RO membrane on desalination performance of seawater are considered first. The MD results show that the salt rejection increases with the rotation speed of porous membrane increasing while the water flux initially increases and then decreases . Meanwhile, the interfacial slip velocity increases linearly with angular velocity increasing. On the other hand, the increasing thickness of porous graphene membrane can enhance the selectivity and reduce the permeability of water molecules. As expected, the tri-layered porous graphene RO membrane can achieve high salt rejection at low interfacial slip velocity. In order to ensure high selectivity and energy conservation and efficient, the pore structure of the porous graphene RO membrane is optimized. The results show that the optimized nanopores can increase the water flux significantly, whereas the salt rejection is not changed appreciably. It is found that the pore size of the innermost layer membrane near the feed region has the most significant effect on the water flux. The water flux increases sharply with the increase of pore diameter and the salt rejection remains totally higher than 80%. Moreover, the RO membrane with a special Type 3 structure exhibits excellent performance in seawater desalination, specifically, the ultrahigh water flux reaches 20029 L·cm<sup>–2</sup>·d<sup>–1</sup> and the super salt rejection arrives at 94%. The research results further clarify and verify the mechanism of the temporal selectivity in RO process, and improve the water flux under the condition of the same membrane thickness by designing gradient hole. The findings can conduce to the in-depth theoretical understanding of porous graphene-based membranes and designing and developing the large-scale seawater desalination devices and water filtration equipment.

It is well known that reverse osmosis (RO) is the leading desalination technology. As an energy intensive technology, the exploitation of renewable energy sources (RES) to power RO systems is a attractive option. A strategy to take advantage of all the available energy of an off-grid renewable system is to work with the RO system under variable operating conditions. This implies additional challenges in terms of water production and permeate quality, among others. Boron rejection is one of the main concerns in seawater RO (SWRO) systems. The aim of this work was to evaluate the performance and boron rejection of a single-stage SWRO system with 7 membrane elements per pressure vessel under variable operating conditions. The initial permeability coefficients of two SWRO membranes (TM820L-440 and TM820S-400) were calculated from experimental data of a full-scale SWRO desalination plant. These coefficients and the characteristics of the membranes were introduced in a simulation algorithm to estimate the behavior of the SWRO system. The results show that, compared with the TM820S-400 membrane, the TM820L-440 performed better in terms of boron rejection in the form of boric acid, but worse in terms of water production. When RES-powered SWRO systems are designed to work under variable operating conditions, consideration needs to be given to the safe operation window in terms of boron concentration in the permeate and to variation of the permeability coefficient of the membranes.

Recent advances in the development of nanoporous graphene (NPG) hold promise for the future of water supply by reverse osmosis (RO) desalination. But while previous studies have highlighted the potential of NPG as an RO membrane, there is less understanding as to whether NPG is strong enough to maintain its mechanical integrity under the high hydraulic pressures inherent to the RO desalination process. Here, we show that an NPG membrane can maintain its mechanical integrity in RO but that the choice of substrate for graphene is critical to this performance. Using molecular dynamics simulations and continuum fracture mechanics, we show that an appropriate substrate with openings smaller than 1 μm would allow NPG to withstand pressures exceeding 57 MPa (570 bar) or ten times more than typical pressures for seawater RO. Furthermore, we demonstrate that NPG membranes exhibit an unusual mechanical behavior in which greater porosity may help the membrane withstand even higher pressures.

Mechanical properties of water desalination and wastewater treatment membranes

Separating molecular species is an important precursor for various applications. In this work, we have utilized molecular dynamics (MD) simulations to examine how pore radius and structure affect the separation process. We show from MD simulations that 2-D graphene sheets with designed sub-nanometer pores can efficiently separate the Kr-85 radionuclide gas from an N2/O2 mixture. Three species of gases (Kr-85, N2 and O2 were considered in the simulation box in which different sizes and geometries of pores were modeled on the graphene sheet. The ( $ 30\times 30\times 80$ A^3 simulation box contains a nanoporous graphene membrane in the middle of the box and two fixed walls with equal distances on both sides of the nanoporous graphene. The results revealed that Kr-85 separation was improved by using an optimized pore structure. It was also found that the Kr-85 gas radionuclides could be completely separated from nitrogen and oxygen molecules in the pore-7 configuration. Restriction of the molecular orientation largely prohibited the permeation of nitrogen molecules. It was also found that nitrogen was more strongly adsorbed onto the membrane than oxygen, while krypton was not adsorbed.

Despite the widespread use of reverse osmosis (RO) membranes in water desalination, the role of solute-membrane interactions in solute transport remains complex and relatively not well understood. This study elucidates the relationship between solute-membrane electrostatic interactions and solute permeability in RO membranes. The transport of salt and neutral molecules through charged polyamide (PA) and uncharged cellulose triacetate (CTA) RO membranes was examined. Results show that salt rejection and salt permeability in the PA membrane are highly dependent on the solution pH due to the variations of membrane charge density and the Donnan potential at the membrane-solution interface. Specifically, a higher salt rejection (and hence lower salt permeability) of the PA membrane is observed under alkaline conditions compared to acidic conditions. This observation is attributed to the enhanced Donnan potential at higher solution pH, which hinders co-ion partitioning into the membrane. In contrast, for salt transport through the CTA membrane and neutral solute transport through both membranes, solute permeability is independent of the solution pH and solute concentration due to the negligible Donnan effect. Overall, our results demonstrate the important role of solute-membrane electrostatic interactions, combined with steric exclusion, in regulating solute permeability in RO membranes.